Infrared radiation filament and method of manufacture

ABSTRACT

An improved IR radiation source is provided by the invention. A radiation filament has a textured surface produced by seeded ion bombardment of a metal foil which is cut to a serpentine shape and mounted in a windowed housing. Specific ion bombardment texturing techniques tune the surface to maximize emissions in the desired wavelength range and to limit emissions outside that narrow range, particularly at longer wavelengths. A combination of filament surface texture, thickness, material, shape and power circuit feedback control produce wavelength controlled and efficient radiation at much lower power requirements than devices of the prior art.

This application is continuation of the application Ser. No. 08/511,070,filed on Aug. 03, 1995, now abandoned.

FIELD OF THE INVENTION

The field of the invention is electro-optical radiation sources and amethod of production of a specifically tuned radiation source. The focusof the invention is a novel filament contained within a packagedradiation source device, configured to be a component in aninstrumentation application. The specific application and embodimentdescribed is an infrared radiation source for use in variouscalibration, reference and measurement instruments; but the filamentcomponent and the method of wavelength tuning that component in themanufacturing process may be applied widely to a variety of otherradiation emission requirements.

BACKGROUND OF THE INVENTION

The tradeoffs and requirements of radiation sources for electromagneticand optical radiation sources, and in particular the use of enclosedelectrically-excited filaments, have been the subject of development forover 100 years. As this development addressed more narrow and specificradiation requirements of controlled wavelength emission for accuracyand precision, power efficiency requirements for economy, and lossreduction and temperature control, the problems involved in design andmanufacture of suitable radiation sources have become correspondinglymore complex.

A particular application environment that has received a great deal ofinquiry is the area of infrared radiation, which is efficiently usefuland necessary in a variety of measurement and detection instrumentation.Many such applications are limited in power, space and cooling abilityand require efficient illumination within a limited spectral band. Someconsiderations of this environment and difficulties of emitter designare discussed in U.S. Pat. No. 3,875,413 to Bridgham for InfraredRadiation Source, which particularly recognizes the difficulty ofachieving stability and control of temperature and emission wavelengthin a thin, flat, electrically heated radiator.

Temperature stability has been a particular development objective oftraditional IR sources for calibration and measurement applications,which rely on steady state heating of an object with relatively largethermal mass. This in turn requires a long turn-on and settling time forstable operation and produces a large amount of waste heat.

As will be seen in the following descriptions, the invention may becompared favorably as an improvement over many previous radiationsources and could usefully replace such traditional reference emissionsources as wire filament bulbs, LEDs, lead salt lasers, and rare earthoxide line emitters in measurement applications. Although these narrowband emitters produce isolated line radiation, they can only be tunedwith difficulty and over narrow ranges. Incandescent sources typicallyproduce a radiation spectrum described by the Planck curve with verylittle of the total radiation in the desired band for a particularmeasurement. Specifically, again in the infrared field, sources of theprior art include developments such as pulsed radiation sources using athin plate form of radiation filament.

The prior art generally teaches the necessity of a thin plate elementfor radiation cooling, the '413 patent referenced above, for instance,specifying 1-2 μm. U.S. Pat. No. 5,220,173 to Kanstad for PulsatingInfrared Radiation Source proposes a formula for required thinness. The'173 patent proposes that thin flat plate elements will efficientlyradiate in the IR range as the low mass of the thin material willradiate greater heat than stored thermal energy delivered by a pulseddriving circuit, and predicts the thinness of material necessary toproduce this effect at the 1-2 micron range. As the focus of the priorart is on radiation source thinness for cooling effect, problems ofemissivity, wavelength control and resistance control have beenunaddressed.

SUMMARY OF THE INVENTION

The inquiry leading to the invention sought to examine the prior art andto practically implement an improved IR source. A new approach tofabrication of the filament emitter was required, as manufacture of theemitter described in the prior art proved problematic in that the flatplate configuration of the emitter could not be made to reliably radiatethe desired radiation wavelength range, or to produce radiation outsidethe desired wavelength spectrum with efficient power consumption. Animproved method of temperature and wavelength control was sought, and adifferent direction from merely specifying the thickness of materialused for the emitter was taken, involving texturing the surface of thefilament material to produce a microscale topography on the radiatingsurface that will enhance radiation while providing precise control ofsource temperature and emission wavelengths. As surface textureprimarily produces the efficacious improvements in emissivity, radiativesurface area, and wavelength control, the invention does not depend, asdoes the prior art, on the native properties of the source elementmaterial to achieve the desired optical effects, nor are coatingsnecessary which undesirably increase mass of the radiator.

The design of the radiation filament of the invention seeks to improveor remedy deficiencies noted in the prior art to the effect thatconventional low thermal mass incandescent sources, including metalribbons, thin flat plates and wire coils, which might otherwise bedesirable for use as a filament, suffer from low emissivity and lowelectrical resistance which causes difficulty in assuring that the drivepower warms the radiator and not the leads and contacts. Bysimultaneously improving the emissivity, thinning the source, andincreasing electrical resistance, the present invention overcomes bothproblems.

A principal objective of the invention in its development was to providea practical method of design and manufacture of an incandescentradiation element spectrally tuned to produce high emissivity within anarrow spectral band. More specifically, an infrared radiation sourcewas sought such that the source emits with the efficiency close to thatof an ideal black body in the desired emission band, but which has lowemissivity outside that band. This was achieved by controlling thesource's surface topography on a micron scale.

Another important objective is to utilize existing but untestedtechnologies for fabricating a radiation filament designed for specificwavelength emissions. Another objective is to provide a radiatingemission source that would be stable, essentially self-correcting, andmechanically simple. A specific application objective is to develop ahigh brightness precision controlled infrared spectrum source emitterthat can be packaged with no moving parts and used in ruggedenvironments.

These and other objectives were achieved and put into practice bydevelopment of techniques for modifying the surface characteristics ofthe radiation filament. By producing a random distribution of featuresof controlled size, surfaces were produced with high emissivity forshort wavelengths and low emissivity for long wavelengths. By making thefeature sizes very uniform, surface emissivity spectra were exhibited insample materials with a sharp long wavelength cut-off, and refinementsin feature size produced adjustments to the exact wavelength of thecut-off point.

A surface that can be produced with microscopic feature topologytailored to produce specific emitted frequencies when electricallystimulated proved practical as various texturing methods--mechanical,chemical, electro-chemical and particle bombardment--were examined. Forany choice of material, differing feature patterns were produced under avariety of texturing methods and variables involved in application ofthose methods, as will be particularly described later in discussion ofthe preferred embodiment. Texturing by any of these means produces apattern of relatively long "fingers" or peaks and valleys that not onlyincrease the radiating surface area remarkably, but also produceinterferences and reinforcements in the interstices that provide highemissivity at wavelengths comparable to the size of the surfacefeatures.

The emissions produced by the textured material surface when stimulatedwill cutoff at the long wavelength end of the desired measurement band.Thus a reproducible sample of material textured with adjusted processvariables is achieved and is defined by the optimum texturing processthat will produce the target radiation spectrum. While the firstembodiment was designed for the IR application range, similarlypredicted texturing processes for a specific surface feature densitywould produce the same controlled emissivity for any target wavelengthin a calculated relationship between surface feature density and desiredradiation wavelength. Various surface modification techniques are knownto produce a range of feature densities related to variables of thematerial surfacing methodologies and may be usefully employed to producethe calculated wavelength emissions in the materials which are discussedherein.

As the discussion of the preferred embodiment illustrates, a practicalmethodology for practice of the invention is to use a directed energyprocess in the form of an ion beam mill to texture the surface of ablank wafer of material to the surface topology that will preciselyradiate the design wavelengths and very little of other wavelengths.This technique also usefully reduces the thermal mass of the material asit is textured. However, many other texturing means could be usefullyemployed, or alternative texturing means used that may produce surfaceeffects not achievable in others. Such alternatives include chemicalbaths, electro-chemical immersion, and various enhancements to energybeam bombardment methods, as well as mechanical abrasion.

While the discussion below discusses primarily ion beam bombardment, nolimitation to this fabrication method is implied, as any means oftexturing will produce modified emission characteristics in a suitablematerial sample. Similarly, although the investigation that produced thepreferred embodiment for IR applications identified titanium foil as anappropriate filament material, many other metal foils, thin nonmetallicand semiconductor materials, and glasses may be textured using thesemethods to modify and control their emissive properties. Metal foils arefound to be particularly adaptable to the techniques of ion beamtexturing, as the foils may be formed as self-supporting filaments in athinness order of a few microns in order that the temperature of thefilament material changes very rapidly in response to changes in inputpower. This responsive temperature rate usefully allows real-timefeedback and control of the source temperature, which is particularlyuseful in applications that require a real-time reference for infraredintensity.

Thus it is possible using the techniques described herein to produce adramatically improved infrared radiation device, including precisespectral tailoring and short warm-up time, that essentially eliminatesparasitic heat which warms the optical train, instrument enclosure anddetector and that causes thermal drift and resulting loss of precision.Because the textured metal foil material is so thin and because it isformed into a folded-path serpentine shape, the filament so formedexhibits high resistance compared to incidental resistance in themounting and drive circuit, assuring that the drive power warms theradiation source and not the leads and contacts. The serpentine shape isespecially useful to increase the electrical resistance and the surfacearea available to radiate in a resistive ribbon format withoutintroducing local temperature non-uniformity ("hot spots") or sharpcomers that may promote stress fractures.

The invention takes a novel approach to emitter element design thatprovides previously unachieved levels of wavelength tailoring to achievemajor efficiencies in output radiation, power consumption and wastelimitation. While the improvement of application in the infraredinstrumentation environment is the inquiry that yields this invention,the configuration design and fabrication methodology described may beapplied in many other applications requiring very efficient andcontrolled electromagnetic and optical emissions.

One goal of the invention is to provide a compact miniature IR referencesource to minimize parasitic heating of associated optics in a systemwith stringent size, weight and waste-heat rejection constraints. Theinvention achieves this objective by enabling an IR source whichincludes real time feedback and control to maintain temperaturestability. Temperature control requires that the source temperaturefollow changes in input power with adequate speed. In practice, thismeans that the source must be in radiative equilibrium with the inputdrive power equal to the power radiated out. While it is at temperature,the source must change temperature by an amount significant to themeasurement on a time scale which is loosely bounded by electricalsampling time for the drive pulse on the fast end, and sharply boundedby the characteristic response time for the infared detector on the slowend, which control criterion may be expressed as a temperature slewrate. For one application, the source was required to maintain 0.5Kstability for an IR system with 1 m-sec sampling time, using a controlcircuit capable of 50 μsec sampling time. This required achieving slewrates of approximately 500 to 10,000 deg K/sec.

Since the device is operating in radiative equilibrium, the device willradiate out heat at a rate proportional to its emissivity and surfacearea. Temperature change, then, is governed by the amount of heat storedin the device per degree per unit area available to radiate. The totalstored heat per degree is given by AtCp (where A is the unit area, t isthe thickness, C is the specific heat, and p is the mass density); theheat stored per degree per unit area thus being tCp. Accordingly,##EQU1##

For the textured titanium sources described as the preferred embodimentof the invention, the final source thickness achieved is on the order of2-10 μm. For a source operating at 950K, a temperature slew rate may becomputed: using a specific heat of 0.523 Jg⁻¹ K⁻¹, an emissivity of 1and a material density of 4.5 g/cm³, this predicts a temperature slewrate in the range from 2×10³ K sec⁻¹ to 10⁴ K sec⁻¹, well within thedesired range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of the radiation emitter showing placement ofthe textured filament;

FIG. 2 is a plan view of a thin material blank showing an imposedcutting pattern for serpentine shaped filament production;

FIG. 3 is a circuit diagram for delivering power to the radiationfilament device, illustrating a feedback loop for precise temperaturecontrol and stability;

FIGS. 4A1 and 4A2 show two electron micrograph samples with surfacefeatures of metal foil after texturing;

FIG. 4B is a plot of emission frequencies associated with each of twoexample textured surfaces; and

FIG. 5 is a representational view of an ion beam bombardment source andprocessing chamber.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, FIG. 1 illustrates in exploded view thecompact configuration of the radiation emitter component as it may beadapted for ready use by mounting in an instrument or on a circuitboard. A cylindrical can-like cover (1) contains a closely fitted window(2) of a material suitably transparent or transmissive to the desiredradiation spectrum of the instrument. As an instrument designed tooperate in infrared frequencies is discussed here, the window materialwas formed of a sapphire which is not only transparent to IR radiationbut is suitably durable in demanding environments in which theinstrument may be installed. The radiation filament (3) is supported atfilament fittings (3a), (3b) and within the can on two upset pins (4),each pin further extended to form electrical leads (6) inserted throughcan floor (7). The filament (3) is securely suspended in the can restingon elevating shoulders (4a), (4b) on the pins (4) and secured by washers(5) such as iconel clamping washers which further enable laser welding.The can configuration may be conveniently sealed by a weld at thejunction of can top (1) and floor (7) and by a seal around the window(2). The can may also filled with an inert gas, if desired, to retardcorrosion of the filament (3).

FIG. 2 shows that the filament (12) may be fabricated from a sheet orblank (10) of suitable material, such as a thin metal foil. In IRapplications, titanium foil is suited to tuning for the applicablefrequency range. The blank (10) is on the order of two inches square andnumerous filament shapes may be laid out on a grid pattern (11), eachfilament (12) laid out as a flat shape that can assume a wide variety ofdesigns for specific objectives, such as: to provide a folded ribbonlength to increase resistive path and rounded closely spaced curves toprovide rigidity and uniform electrical heating with no hot spots; andto provide construction details like the measured support peg fittings(3a), (3b) at the ends of the filament (3) shown in FIG. 1. Theserpentine or multiple folded curve shape of the filament as shown isdesigned to meet all these criteria for this application. Aftertexturing the entire blank (10) as discussed below, for example by ionbeam bombardment, the individual filaments are cut from the blank (10)by stamping or by such precision cutting as a computer-controlled wireEDM method.

As many other prior art devices have illustrated, a pulsed current isdesirable to limit the emission to the minimum necessary time, todissipate heat in the off cycle, and to coincide with instrumentfunction timing; and the high temperature slew rate (necessary forcontrol) allows this pulsed operation. Further, a power circuit may alsoinclude a feedback loop to ensure temperature stability by adjustingdrive power, for instance, to accommodate changes in emitter temperaturewhich may cause temperature and wavelength drift. A typical feedbackcontrol power circuit (17) is illustrated in FIG. 3; and circuit (17)embodies a control strategy to take advantage of the high temperatureslew rate available with the radiation source of this invention bymonitoring the electrical drive signal, either by measuring currentthrough the radiation element or the voltage drop across the element, orboth. In circuit (17), a commercially available PIC microcontroller(17a) commands a 12-bit D/A converter (17b) to output a voltage signalproportional to the desired current through the source and a precision,low-noise operational amplifier (17c) continuously adjusts the gatevoltage of a power MOSFET (17d) to achieve this current through thedevice. Depending on the desired temperature, current through the deviceis on the order of several hundred mA (200 mA at 500K, for example)while it is on. At 500K, the rate of temperature change with current isapproximately 0.8 mA/K, so that the 12-bit dynamic range of the A/Dconverter (17b) is more than adequate to achieve the specifiedstability.

Power requirements of the filament configuration according to theinvention are lower because the suppression of wavelength radiationoutside the target spectrum provides significant improvement inconversion efficiency compared to nontextured filaments. The texturedmaterial of the filament enables the active feedback control of sourcetemperature provided by the illustrated circuit (17), as thermal mass issmall enough that the source reaches radiative equilibrium quickly (on atime scale of 100 μsec/deg K.) so that available A/D chip circuitelements can readily follow it.

As texturing of the filament is identified as a critical focus of theinvention, that texturing is illustrated in physical dimensions by thescanning electron micrographs of FIGS. 4A1, 4A2, taken from samples ofalloy foils that had been bombarded in an ion beam mill for thispurpose. It may be clearly seen in the two SEM depictions of FIGS. 4A1,4A2 that surface feature size and character not only vary dramaticallyfrom sample to sample, but are remarkably uniform within each sample. Asvariables of the mill are adjusted, differing feature dimensions andspacing are achieved, such as the visible differences between the topdisplayed sample of FIG. 4A1 and the bottom sample of FIG. 4A2. Bothsamples were milled to about five microns thickness with the resultingvertical "fingers" extending upward from the lower surface for much ofthat height; valleys and spaces are visible and form larger intersticesin the top sample and a tighter pattern on the bottom sample. It canclearly be envisioned from the texturing patterns that the emittingsurface area of a filament so formed is much greater than a smoothmaterial blank by several multiples, and that over the entire surfacethe emission spectrum would be regularized by patterns of interferenceand reinforcement.

Two such samples (but not necessarily the same samples as depicted inthe SEM photographs of FIGS. 4A1, 4A2) were tested to determine thefrequency emission characteristics, shown on the plot of FIG. 4B. Theemitted wavelength exhibited by each sample against emissivity shows aprimary range of about 5-15 microns for one and 10-20 microns for theother. Each of these samples can produce the same controlled response asthey are textured of the same materials and variable milling conditions.Close examination of the SEM photographs of FIGS. 4A1, 4A2 yields ameasure of feature density which correlates with radiative frequenciessuch that the cut-off wavelength occurs at approximately 2π times themedian feature density. Of course, fine adjustments in the ionbombardment or other texturing processes will result in fine adjustmentsto the resulting feature density, thereby fine tuning the radiativewavelength range.

The preferred ion beam texturing process is illustrated by the schematicof FIG. 5 showing a plasma source used as an ion beam mill (20) inrepresentational form. The sample (22), for instance the blank (10) ofFIG. 2, is supported by sample holder (23). A vacuum is disposed in theprocess chamber (29) with a suitable pump (24), and an ion gauge (25).The ion beam--which originates in a plasma formed by magnetron (33) viacoupler (32), wave guide (31), and permanent magnet (28)--typicallypasses through an extraction grid (26) on the way to the object point atwhich sample texturing occurs by incidence of the beam at a controlledangle and strength. Another variable effect is produced by the ion beammill configuration as an alternating current field that will alsocontrol ion extraction as it surrounds the blank and seed source. Yetanother effect may be obtained by introduction of DC bias, which may beapplied as a control mechanism of ion extraction which in turn affectstexturing. Inert gas plasmas such as argon may be used as an immersionmedium within the mill.

A number of variables and supplemental techniques can vary the texturingeffect produced by the ion beam mill. For instance impurities may beintroduced to the sample by inclusion of a seed mesh, and in theapplication of the preferred embodiment it was found that inserting atantalum mesh produced desirable texturing effects in the ion beammilling process and that the effects could be varied for tuning byapplying variable bias voltages to the mesh. Beam current, and seedingrate variables also affect surface finish. Control of surfacetemperature of the blank or regulating vacuum chamber oxygen partialpressure during bombardment affects resulting feature size. Further,other ion sources have been used in the milling process such as aKaufmann type ion beam sputtering system which may produce similarlyuseful texturing effects.

While the functioning of the ion milling process are well known for avariety of purposes including metals texturing, the application of thispowerful energy directed process to fabricating precisely tunedradiation emitters is unexpectedly efficacious and resolves the problemsof the prior art outlined above. Similarly, the texturing could beaccomplished in the same iterative sample tuning process by othermethods such as chemical etching, electro-chemical immersion, or otherforms of energy beam milling.

The process of texturing the blank by any method, but in particular byion beam bombardment, will not only texture the sample but desirablyreduce the mass and thickness significantly in the process by 50% ormore from its initial untextured mass. For instance the titanium foil ofthe preferred embodiment was reduced from 12 microns to 6 microns as itwas tuned to the target emission spectrum. Note however that the endthickness is not the determining factor of achieving the emissionwavelength tuning, since reducing mass is not critical and since tuningis primarily a function of the surface texture.

What is claimed is:
 1. A radiation source, comprising:a filament forproducing infrared emissions tailored to a selected infrared wavelengthspectrum when electrically stimulated, said filament having a texturedsurface with features therein that are appoximately sized to saidselected infrared wavelength spectrum, and means for mounting thefilament to electrical terminals.
 2. The radiation source of claim 1wherein said features are regularly distributed about said surface andextend outwardly from said surface.
 3. The radiation source of claim 1wherein said filament is shaped to increase electrical resistance whilemaintaining sufficient rigidity and geometry to be supported within ahousing.
 4. The radiation source of claim 3 wherein said filament is cutto a multiply folded curved shape.
 5. The radiation source of claim 1wherein said wavelength spectrum of said filament is tuned to aninfrared radiation range.
 6. The radiation source of claim 1 whereinsaid filament comprises titanium foil.
 7. The radiation source of claim1 further comprising:an electric power circuit connected to saidfilament to provide metered current to excite said filament, and afeedback control circuit operatively connected to said power circuit tomonitor resistance of said filament and to adjust current to saidfilament, thereby controlling temperature of said filament.
 8. Theradiation source according to claim 1, further comprising a housing tocontain and support the filament.
 9. The radiation source of claim 1,wherein the features are sized to between about two and ten microns. 10.The radiation source of claim 1, wherein the features are substantiallyuniform in size such that the emissions have a cut-off wavelengthgreater than the size.
 11. The radiation source of claim 10, wherein thecut-off wavelength is approximately 2π times the size.
 12. The radiationsource of claim 1, wherein the features comprise peaks and valleys whichincreases radiative surface area.
 13. The radiation source of claim 1,wherein the features are formed by ion beam bombardment.
 14. Theradiation source of claim 1, wherein the filament comprises metal foil.15. The radiation source of claim 14, wherein the metal foil comprisestitanium.
 16. The radiation source according to claim 1, wherein thefilament has a thickness of approximately five microns.
 17. Theradiation source according to claim 1, further comprising a package forenclosing the filament, the package having a window substantiallytransparent to the spectrum and two terminals extending through thepackage, the filament being mounted on the terminals within the packagewherein the emissions transmit through the window.
 18. The radiationsource according to claim 1, wherein the features are randomlydistributed about the surface.
 19. A radiation source comprising:afilament and two terminals for supporting said filament, the filamenthaving a surface textured in a pattern tuned to a selected infraredemissivity spectrum and being shaped to include at least one curvebetween said two terminals, electrical leads connected to saidterminals, and p1 a package comprising a floor and a covering can, saidterminals being rigidly fixed to said package and said leads extendingthrough the floor of said package.
 20. The radiation source of claim 19,further comprising a window made from, sapphire.
 21. The radiationsource of claim 19, further comprising two washers of inert material forretaining said filament on said terminals, each of said washers being aweld medium to retain said filament on one of said terminals.
 22. Theradiation source according to claim 19, further comprising a window ofmaterial substantially transparent to the spectrum.
 23. The radiationsource according to claim 19, further comprising an inert gas forfilling the package.
 24. The radiation source according to claim 19,wherein the can is welded to the floor.
 25. The radiation sourceaccording to claim 19, wherein the features are randomly distributedabout the surface.